The present invention relates to a process for forming high quality crystalline refractory materials, particularly gallium (III) nitride (GaN), from solid precursors.
GaN is a material newly available for use in the optoelectronics industry for the fabrication of light-emitting diodes (LEDs) and blue lasers. It is also possible that doped GaN crystals may have utility as semiconductors. A particularly suitable application is the replacement of standard light bulbs in large outdoor displays, traffic lights and street lighting by GaN LEDs. GaN crystals, when properly activated, fluoresce producing a bright blue glow which is about 60 times brighter than the best GaP based yellow-green LEDs and many times brighter than a standard light bulb which it would replace. Further, a GaN LED display would have an operating life far in excess of the standard light bulb.
Currently, bulk quantities of high purity, polycrystalline gallium nitride are not available. Current techniques to produce such materials require maintaining reactants at high temperatures and pressures for long periods of time. Prior attempts to manufacture GaN by reacting gallium iodide with lithium nitride, without highly elevated pressures, which appears to be a suitable approach, produces elemental Ga, nitrogen and Lil and not GaN.
An alternative approach would be to find a way to lower the reaction temperature. Inert additives such as various salts have been used to lower the temperatures of other metathesis reactions. However, this leads to less crystalline products. (Wiley, J. B. and Kaner, R. B., Science, 2255, 1093 (1992)). Inert salt additives (e.g. NaCl, LiCl, Lil, etc.) do not participate in the chemical reaction. Their presence lowers the temperature of the reaction by acting as a heat sink and diluting the reactants. Research has shown that addition of inert salts slows the propagation of reactions but also results in unacceptable product crystallinity. When too much salt is added, specific to each particular reaction, the reaction lacks the energy to self-propagate and will only react if externally heated. In theory, addition of 6.2 or more moles of lithium chloride (a common inert salt additive) should lower the reaction temperature enough (.ltoreq.940.degree. K.) to favor the formation of gallium nitride under ambient pressure. Tests show that the addition of an inert salt in the predicted amounts does lower the temperature of the reaction; however GaN does not crystallize as a product.
Rodriguez, et al. used NH.sub.4 Cl in the SHS synthesis of Si.sub.3 N.sub.4 to aid in the nitridization of Si. (M. A. Rodriguez, N. S. Makhonin, J. A. Escrina, I. P. Brovinkaya, M. I. Osendi, M. F. Barba, J. E. Iglesias, J. S. Moya, Adv. Mater., 7, 8 (1995)). This process consisted of a Si powder, seed crystals of .beta.-Si.sub.3 N.sub.4 and NH.sub.4 F compacted and detonated under 100 atm nitrogen overpressure for 30 minutes to produce crystalline .beta.-Si.sub.3 N.sub.4.
U.S. Pat. No. 5,453,407 is directed to the formation of nitride ceramic powders by mixing selected powdered metals, including Si, B, Al, Zr, Ti, Cr, and V, with a solid state nitride, an ammonium salt and an igniting agent in nitrogen at 1 to 10 atm. to form a powdered nitride of the metal. The igniting agent is identified as a critical part of the procedure. However, there is no suggestion this process is suitable to form GaN.
Thus there is a need for a low cost, rapid process to produce large quantities of powdered crystalline materials, particularly GaN, for use in such applications as lighting, signal displays, and flat screen displays for computers and television screens.